One area of nanotechnology is to develop chemical building blocks from which hierarchical molecules of predicted properties can be assembled. An approach to making chemical building blocks or nanostructures begins at the atomic and molecular level by designing and synthesizing starting materials with highly tailored properties. Precise control at the atomic level is the foundation for development of rationally tailored synthesis-structure-property relationships which can provide materials of unique structure and predictable properties. This approach to nanotechnology is inspired by nature where, for example, from twenty common amino acids found in natural proteins, more than 105 stable and unique proteins are made.
Nanotechnology has also been described by K. Eric Drexler in Engines of Creation as “the knowledge and means for designing, fabricating and employing molecular scale devices by the manipulation and placement of individual atoms and molecules with precision on the atomic scale.” A quest of nanotechnology is to prepare molecular architectures capable of performing on a nanometer scale functions normally observed for large-scale constructs. For example, rotaxanes and polyrotaxanes are molecules that are interlocked, but not chemically bound to one another, which act like nano-machines. In other examples, carbon nanotubes and similar constructs have been created which may function as molecular scaffold units, or as transport channels, storage units, or encapsulators for various atoms and molecules. The use of biological processes is also being studied as an approach to the assembly of non-biological nano-devices. For example, U.S. Pat. No. 5,468,851 discloses building various structures from polynucleotide segments.
In U.S. Pat. No. 5,876,830 an approach to the construction of macromolecular structures by coupling molecular modules using connectors, spacers, and binders is described. The modules were adhered to a surface and reacted to form grids or nets on the surface.
International Patent Application WO 01/27028 A1 describes structural sub-units or synthons which could be used to prepare molecular nanostructures, machines and devices. Synthons used were closo-carboranes, which are rigid polyhedral structures, selected for their availability and requisite substitutional control and structural diversity.
Some aspects of nanotechnology are described in Chemical Reviews, 1999(7).
One field that will benefit from nanotechnology is filtration using membranes. Conventional membranes used in a variety of separation processes can be made selectively permeable to various molecular species. The permeation properties of conventional membranes generally depend on the pathways of transport of species through the membrane structure. While the diffusion pathway in conventional selectively permeable materials can be made tortuous in order to control permeation, porosity is not well defined or controlled by conventional methods. The ability to fabricate regular or unique pore structures of membranes is a long-standing goal of separation technology.
In one example, the formation of selectively permeable membranes of monomolecular thickness was described by Hendel, et al., Journal of the Amer. Chem. Soc., 1997, 119:6909-18, who reported preparation of calix[6]arenes and their deposition as Langmuir-Blodgett films on a porous poly[1(trimethylsilyl)-1-propyne] substrate, where the calix[6]arene molecules are not coupled or bound to each other in the film. A selectively permeable membrane was described for which the ratio of the normalized flux of helium gas to nitrogen gas was found to significantly exceed the conventional Graham's law prediction.
Resistance to flow of species through a membrane may also be governed by the flow path length. Resistance can be greatly reduced by using a very thin film as a membrane, at the cost of reduced mechanical strength of the membrane material. Conventional membranes may have a barrier thickness of at least one to two hundred nanometers, and often up to millimeter thickness. In general, a thin film of membrane barrier material can be deposited on a porous substrate of greater thickness to restore material strength.
Membrane separation processes are used to separate components from a fluid in which atomic or molecular components having sizes smaller than a certain “cut-off” size can be separated from components of larger size. Normally, species smaller than the cut-off size are passed by the membrane. The cut-off size may be an approximate empirical value which reflects the phenomenon that the rate of transport of components smaller than the cut-off size is merely faster than the rate of transport of larger components. In conventional pressure-driven membrane separation processes, the primary factors affecting separation of components are size, charge, and diffusivity of the components in the membrane structure. In dialysis, the driving force for separation is a concentration gradient, while in electrodialysis electromotive force is applied to ion selective membranes.
Thus, what is needed is an approach to making chemical building blocks or nanostructures from starting materials with tailored properties.